Time of flight ranging systems find use in level measurements applications, and are referred to as level measurement systems. Level measurement systems determine the distance to a reflective surface (i.e. reflector) by measuring how long after transmission energy, an echo is received. Such systems may utilize ultrasonic pulses, pulse radar signals, electromagnetic waves, or other microwave energy signals.
Radar and microwave-based level measurement systems are typically preferred in applications where the atmosphere in a container or vessel is subject to large temperature changes, high humidity, dust and other types of conditions which can affect propagation. To provide a sufficient receive response, a high gain antenna is typically used. High gain usually translates into a large antenna size with respect to the wavelength.
Two types of antenna designs are typically found in microwave-based level measurement systems: rod antennas and horn antennas. Rod antennas have a narrow and elongated configuration and are suitable for containers having small opening/flange sizes and sufficient height for accommodating larger rod antennas. Horn antennas, on the other hand, are wider and shorter than rod antennas. Horn antennas are typically used in installations with space limitations, for example, vessels or containers which are shallow.
The level measurement instrument or device comprises a housing and an antenna. The level measurement instrument is mounted on top of the container or vessel and the antenna extends into the vessel. The level measurement instrument is typically bolted to a flange around the opening of the container, i.e. the process connection, and attached to the process connection are the antenna and the housing. The housing holds the electronic circuitry. The antenna extends into the interior of the vessel and is connected to a coupler which is affixed to the housing. The antenna is electrically coupled to the electronic circuit through a coaxial cable. The coaxial cable has one terminal connected to the antenna coupler and the other terminal is connected to a bidirectional or input/output port for the electronic circuit. The antenna converts guided waves into free radiated waves, and is reciprocal, i.e. also converts the free radiated waves into guided waves. The antenna is excited by electromagnetic (i.e. radio frequency) waves or energy or microwave signals received through the coaxial cable from the circuit and transmits electromagnetic waves or energy into the vessel. The antenna couples the electromagnetic waves that are reflected by the surface of the material contained in the vessel and these waves are converted into guided electromagnetic signals which are guided by the coaxial cable (i.e. waveguide) to the circuit.
For safety reasons, for example, intrinsic safety requirements under the EN50020 standard, the radar level measurement devices are required to provide galvanic or DC isolation between the measured process (i.e. the vessel and material interface) and the electronic circuitry in the device. Because the antenna is in contact with the process, the requirement for galvanic isolation is applied between the cable powering the antenna and the electronic circuitry.
In the art, galvanic isolation is an important design element for level measurement apparatus. To be effective, galvanic isolation mechanisms must provide the required isolation, i.e. DC blocking, while minimizing transmission losses and/or reflections. Accordingly, there remains a need for improvements in galvanic isolation mechanisms.